In 1996, a method was reported for utilizing biomolecules, such as DNA, and their molecular recognition properties to guide the assembly of nanoparticle building blocks modified with complementary recognition elements into functional materials.1 These materials have found wide application in the development of highly sensitive and selective diagnostic methods for DNA.2 This material synthesis approach has been extended to a wide range of biomolecules, including peptides and proteins,3 and a modest collection of nanoparticles including gold and semiconductor quantum dots.4-9 In each case, when a new nanoparticle composition is designed, new modification methods must be developed for immobilizing biomolecules on the surface of the particles of interest. This approach has been extensively utilized but with limited success. The methods for modifying gold nanoparticles have now been optimized and generalized for a wide range of particle sizes and surface compositions, including spheres and rods.1,2,4,10 Gold particles are particularly easy to modify because they are often stabilized with a weakly binding layer of charged ligands (e.g. citrate) that can be replaced with molecules with chemical functionalities that bind more strongly (e.g. thiols, amines, and disulfides) to their surfaces than these ligands. The CdSe and CdS quantum dots have proven more difficult to modify because they have a surfactant layer that is very strongly bound to their surfaces and, consequently, difficult to displace.5 No successful routes have been developed for creating stable oligonucleotide conjugates with silver nanoparticles, primarily because they tend to chemically degrade under conditions used to effect DNA hybrization. A major advance would be to devise a method for designing particles with the physical properties of a chosen nanoparticle composition but the surface chemistry of gold. Herein, a low temperature method is provided for generating core/shell particles consisting of a silver core and a non-alloying gold shell that can be readily functionalized with oligonucleotides using the proven preparatory methods for pure gold particle oligonucleotide conjugates.2d Moreover, the novel nanoparticle composition can be used to access a colorimetric detection system distinct from the pure gold system.2a,2d 